The most common solid fertilizers are water-soluble compounds (organic and inorganic compounds, such as urea, potassium sulfate, potassium nitrate, ammonium nitrate, ammonium sulfate, ammonium phosphate, etc.). However, the high solubility of such compounds reduces their efficiency, as they are drained by rain or irrigation water too quickly for the plants to utilize them efficiently. Moreover, the fertilizers lost to the surroundings cause environmental problems (e.g. nitrates poison the drinking water system, phosphates cause eutrophication of rivers and lakes, etc.).
Two general methods were suggested in the art to overcome the above disadvantages, namely:
a. To use substitutes for the said common fertilizers, which are slow-release fertilizers (SRF), such as urea-formaldehyde, oxamide, melamine, etc., which however are much more expensive than the usual fertilizers; and
b. To use physically modified SRF, which are prepared by mixing and/or coating and/or encapsulating/microencapsulating the water soluble fertilizers with waxes, sulphur, synthetic polymers, cements (e.g. Portland cement), organic acids, etc. These modifications lead to products of better cost-performances than (a), but which are still not good enough to replace the regular fertilizers in common uses.
The literature deals quite extensively with both the abovementioned approaches. A general review of slow-release fertilizers SRF (CEH Marketing Research Report--"Controlled Release Fertilizers"; by S. P. Landels with A. leder and N. Takei; 1990 by the Chemical Economics Handbook--SRI International) presents a comprehensive view of the two general methods that were mentioned above. This review supports the view that no technology for the production of SRF is available in the examined art, which could lead to replacement of most of the water-soluble fertilizers in use. Moreover, it is clear from the economical discussion that only very simple production methods and very inexpensive reagents may be used to convert the fertilizers into successful and widespread SRF.
EP 0276179 discloses a method of coating water-soluble fertilizers with water-insoluble metal salts of organic acids containing 6 to 30 carbon atoms. A close examination of this technology shows that it is not useful for practical purposes, as the constituents of the coating, including the paraffin, are too expensive and the whole coating process is too cumbersome and costly.
DE 141519 deals with a technology for preparing SRF sticks for gardening, which are based on a skeleton of sand and calcium aluminate cement. Such products may be used for small scale application in which the cost-effectiveness is not of prime importance, but is not suitable for large scale agricultural and industrial use. U.S. Pat. No. 4,023,955 discloses a method for encapsulating water-soluble fertilizes with hydrated cement. Superficially, this technology looks very promising. However, a close scrutiny reveals the complexity of the operation that is necessary to obtain a useful SRF. Moreover, it requires the use of elastomers which are not necessarily, environmentally friendly or inexpensive. Also, this technology was not in fact implemented to any appreciable extent.
U.S. Pat. No. 5,030,267 describes a technology useful to coat water-soluble fertilizers with specially prepared calcium metasilicate cement, and to turn it into SRF. Indeed, the product is obtained, but the expenses involved in preparing this special cement and the further costs that are associated with the rest of the coating procedure make it impossible to obtain low cost SRF.
DE 145,015 discloses a method for preventing urea from caking on storage by coating it with magnesite cements (oxychloride and oxysulfate). This patent is specific to urea, and does not teach or in any way discloses that this surface-modified urea is also a SRF. Indeed, its product is not a SRF, because the magnesite cements deteriorate quite fast in water and the urea dissolves too rapidly in water. Therefore, this coated urea can not be considered a proper SRF, as will be shown in the examples to follow.
Two Israeli patent applications (Nos. 109825 and 109826) have been filed on May 30, 1994 by the same applicant herein, the specifications of which are incorporated herein by reference. The aforementioned Israeli patent applications describe a method to improve magnesite cements (magnesium oxychloride and/or oxysulfate). This method comprises mixing the following materials: MgO; MgCl.sub.2 and/or MgSO.sub.4 ; water; organic carboxylic acid(s) and/or their salts and/or their acid anhydrides. The resulting cements have been found to be of much improved physical properties and most of all--their stability in water changed dramatically. An extensive comparison of the properties of the improved magnesite cements to those of the regular cements is given in the above patent applications.
The outstanding stability of the improved magnesite cements in water makes them extremely suitable for the preparation of SRF, while the regular magnesite cements do not offer such properties and, therefore, are not suitable for that purpose. The examples provided hereinafter will clearly show the marked differences between the SRF made with the improved magnesite cements and those attempted with the regular cements.
It is an object of the present invention to provide controlled-release fertilizers which can be used effectively for large agronomical crops at relatively low cost.
It is another object of this invention to provide a process for converting water-soluble fertilizers into low cost controlled-release fertilizers.
It is still another object of this invention to provide controlled-release fertilizers including suitable fillers--especially the soil in which the controlled-release fertilizers is intended to be used.
It is a further object of this invention to provide improved controlled-release fertilizers comprising flotation reagents that are specific to brine solutions and the particular water-soluble fertilizer.
It is another object of this invention to provide controlled-release fertilizers made of a blend of two or more water-soluble fertilizers, in the presence of MgCl.sub.2 or MgSO.sub.4 solutions.
It is an object of this invention to provide improved magnesite cement coatings useful as sequestering agents for heavy metal ions that are deliberately added to the fertilizers to broaden their nutrient value.
It is yet another object of this invention to provide controlled-release fertilizers which are essentially environmentally friendly.
All the above and other characteristics and advantages of the invention will be better understood through the following illustrative and non-limitative description of preferred embodiments.